This invention relates to a pneumatic radial tire, which has a belt reinforcement layer consisting of a layer of steel cords and a layer of aromatic polyamide fiber cords having its side ends turned over toward each other, and which is characterized in that the diameter of the cords in the layer of aromatic polyamide fiber cords, the number of these cords in use and the properties of covering rubber are specifically set to thereby improve the durability of the tire, especially, the durability in bent portions of the aromatic polyamide fiber cords.
In general, conventional pneumatic radial tires for passenger vehicles have a carcass layer in which a synthetic fiber such as nylon fiber and polyester fiber is used as material for the cords therein, and a belt reinforcement layer consisting of two steel belt layers in which the cords in one layer cross those in the other. Owing to the presence of these steel belt layers, pneumatic radial tires have improved durability, high-speed stability and steering stability as compared with bias tires. However, such steel belt layers cause vibration which lessens the driving comfort.
In order to eliminate such shortcoming of pneumatic radial tires, there has of late been proposed a tire having a belt reinforcement layer, which has a steel belt layer and a layer of aromatic polyamide fiber cords having its side ends turned over toward each other and which is formed so that the cords in one layer cross those in the other (Japanese patent publication No. 53603/1983). This belt structure provides a better driving comfort than a two-layer steel belt structure, but poses a problem such that separation is liable to occur at these portions of the belt structure which are in the vicinity of the point of bending or turning of the layer of aromatic polyamide fiber cords. This problem arises due to the following facts: Whereas the strength of the aromatic polyamide fiber is very high as compared with that of conventional cord material, the crystallinity thereof is high in its molecular structure and its adhesion affinity with the covering rubber is low. Therefore, when aromatic polyamide fiber cords are used, likely is the separation, a phenomenon in which the cords and the covering rubber are separated from each other.